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Re: Cursor fetch performance issue

From: Andy Colson <andy(at)squeakycode(dot)net>
To: Tony Capobianco <tcapobianco(at)prospectiv(dot)com>
Cc: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Cursor fetch performance issue
Date: 2012-01-24 21:11:22
Message-ID: 4F1F1E7A.4080403@squeakycode.net (view raw or flat)
Thread:
Lists: pgsql-performance
> On Tue, 2012-01-24 at 21:47 +0100, Pavel Stehule wrote:
>> Hello
>>
>> 2012/1/24 Tony Capobianco<tcapobianco(at)prospectiv(dot)com>:
>>> We are migrating our Oracle warehouse to Postgres 9.
>>>
>>> This function responds well:
>>>
>>> pg=# select public.getMemberAdminPrevious_sp2(247815829, 1,'test(dot)email(at)hotmail(dot)com', 'email', 'test');
>>>   getmemberadminprevious_sp2
>>> ----------------------------
>>>   <unnamed portal 1>
>>> (1 row)
>>>
>>> Time: 7.549 ms
>>>
>>> However, when testing, this fetch takes upwards of 38 minutes:
>>>
>>> BEGIN;
>>> select public.getMemberAdminPrevious_sp2(247815829, 1,'test(dot)email(at)hotmail(dot)com', 'email', 'test');
>>> FETCH ALL IN "<unnamed portal 2>";
>>>
>>> How can I diagnose any performance issues with the fetch in the cursor?
>>>
>>
>> Cursors are optimized to returns small subset of result - if you plan
>> to read complete result, then set
>>
>> set cursor_tuple_fraction to 1.0;
>>
>> this is session config value, you can set it before selected cursors queries
>>
>> Regards
>>
>> Pavel Stehule
>>
>>> Thanks.
>>> Tony
>>>
>>>
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>>
>
>
>


On 1/24/2012 2:57 PM, Tony Capobianco wrote:
 > Running just the sql of the function returns only 10 rows:
 >
 > pg=# SELECT m.memberid, m.websiteid, m.emailaddress,
 > pg-#                m.firstname, m.lastname, m.regcomplete, m.emailok
 > pg-#         FROM   members m
 > pg-#         WHERE  m.emailaddress LIKE 'test(dot)email(at)hotmail(dot)com'
 > pg-#         AND    m.changedate_id<  5868 ORDER BY m.emailaddress, 
m.websiteid;
 >   memberid  | websiteid |    emailaddress        | firstname | 
lastname | regcomplete | emailok
 > 
-----------+-----------+------------------------+-----------+----------+-------------+---------
 >   247815829 |         1 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       1
 >   300960335 |        62 | test(dot)email(at)hotmail(dot)com |           | 
    |           1 |       1
 >   300959937 |       625 | test(dot)email(at)hotmail(dot)com |           | 
    |           1 |       1
 >   260152830 |      1453 | test(dot)email(at)hotmail(dot)com |           | 
    |           1 |       1
 >   300960163 |      1737 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       1
 >   300960259 |      1824 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       1
 >   300959742 |      1928 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       1
 >   368122699 |      2457 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       1
 >   403218613 |      2464 | test(dot)email(at)hotmail(dot)com | email     | test 
    |           1 |       0
 >   378951994 |      2656 | test(dot)email(at)hotmail(dot)com |           | 
    |           1 |       1
 > (10 rows)
 >
 > Time: 132.626 ms
 >
 > So, it would seem that's a small enough number of rows. 
Unfortunately, issuing:
 >
 > set cursor_tuple_fraction to 1.0;
 >
 > Did not have an effect on performance.  Is it common to modify this
 > cursor_tuple_fraction parameter each time we execute the function?
 >
 >


So, is getMemberAdminPrevious_sp2() preparing a statement with wildcards?

SELECT m.memberid, m.websiteid, m.emailaddress,
        m.firstname, m.lastname, m.regcomplete, m.emailok
        FROM   members m
        WHERE  m.emailaddress LIKE $1
        AND    m.changedate_id < $2
       ORDER BY m.emailaddress, m.websiteid;

Or is it creating the string and executing it:

sql = 'SELECT m.memberid, m.websiteid, m.emailaddress, '
     ||  ' m.firstname, m.lastname, m.regcomplete, m.emailok '
     ||  ' FROM   members m
     ||  ' WHERE  m.emailaddress LIKE ' || arg1
     ||  ' AND    m.changedate_id < ' || arg2
     ||  ' ORDER BY m.emailaddress, m.websiteid ';
execute(sql);

Maybe its the planner doesnt plan so well with $1 arguments vs actual 
arguments thing.

-Andy



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Subject: Re: Cursor fetch performance issue
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